Unbreak build on FreeBSD (#3304)

* Unbreak build on FreeBSD

```
In file included from src/core/Alloc.cpp:24:
/usr/include/malloc.h:3:2: error: "<malloc.h> has been replaced by <stdlib.h>"
 ^
src/core/Alloc.cpp:65:28: error: use of undeclared identifier 'malloc_usable_size'
    ::operator delete(ptr, malloc_usable_size(ptr));
                           ^
2 errors generated.
```

Non-standard APIs like `malloc_usable_size()` [1] are defined in
`malloc_np.h` on FreeBSD, so use it instead of `malloc.h` there.

[1] https://man.freebsd.org/jemalloc(3)

* Allow Browser HostInstaller to work on FreeBSD and other OS's

* Drop custom GNUInstallDirs cmake module and use cmake's own module

It seems to be an outdated version and cmake generally provides it
itself, so there should be no need for keepassxc to provide its own
custom version.

On FreeBSD this fixes the issue that man pages were installed into
the wrong directory, i.e., `/usr/local/share/man` vs `/usr/local/man`
as per FreeBSD's current packaging policy.

Signed-off-by: Tobias Kortkamp <tobik@FreeBSD.org>

src/core/Alloc.cpp

@@ -18,8 +18,10 @@
 #include <QtGlobal>
 #include <cstdint>
 #include <sodium.h>
-#ifdef Q_OS_MACOS
+#if defined(Q_OS_MACOS)
 #include <malloc/malloc.h>
+#elif defined(Q_OS_FREEBSD)
+#include <malloc_np.h>
 #else
 #include <malloc.h>
 #endif